Serena Williams ended a frustrating season the way she started -- with an injury. Williams strained an abdominal muscle that blunted her powerful serves in a 4-6, 6-2, 6-4 loss to Maria Sharapova in the final of the WTA Championships on Monday night. ``It's extremely disappointing,'' she said. ``I really figured I'd have a good chance at this title.'' Instead, Williams finished the year with a No.